The invention relates to a mechanical transmission for drive wheels, in particular in mobile work machines.
Specifically but not exclusively the transmission is usefully applied to small mobile work machines, wherein the means for propulsion, usually wheels or tracks, are used mainly for moving masses of small proportions as well as for moving the machine itself, quite slowly, from one work position to another (such as in the case of excavators).
For these purposes generally it is sufficient to use extremely simple and cheap traction means, constituted as a rule by a hydraulic rotary geared motor using the same hydraulic circuit that also activates the tool organs of the machine and which functions at constant or minimally-variable rotation speeds.
Prior art mechanical transmissions for the above-described purpose usually equip a gear mechanism comprising a sprocket, solid to the drive shaft, connected through an idler gear to an internally-cogged crown wheel solid to the hub, with the drive wheel being solidly constrained to the crown. The mechanical transmissions have a fixed gear ratio, so the machine has only one speed, which is the same as the relatively low speed needed for it to perform its task or move about from one workplace to another.
The main drawback of the known work machines using such mechanical transmissions is that the machines are not truly able to make long journeys under their own power; their speed would render such journeys epic. Hence the machines are usually loaded onto other, faster vehicles for such journeys.
PCT publication WO 91/0044 teaches a hub incorporating a planetary gear with two transmission ratios, selectable by means of a hydraulic clutch. This two-stage device is a very laborious solution, being neither highly function nor economical.